The invention relates to a method and apparatus for measuring the oxygen concentration in blood. More particularly, the invention relates an esophageal probe having an oximeter sensor for measuring the fraction of hemoglobin (HGB) that is present in the form of oxyhemoglobin (HGBO) and a method of positioning the oximeter sensor in the esophagus.
Oximetry is the measurement of the fraction of hemoglobin (HGB) that is present in the form of oxyhemoglobin (HGBO). When oxygen is taken into the lungs, it is transferred across the alveolar membrane to the blood cells and chemically attaches to hemoglobin molecules (HGB) to form oxyhemoglobin (HGBO). Therefore, the amount of oxygen being transferred from the lungs into the blood stream can be deduced by measuring the fraction of HGB that is present in the form of HGBO. It is important to know the oxygen levels in order to determine the efficacy of cardiac function and perfusion (i.e., delivery of oxygen to tissue).
One method of making this measurement involves a technique called surface oximetry. A finger or earlobe is placed between, on the one side, a pair of light emitting diodes and, on the other side, a pair of photo-transistors. Light having two separate wavelengths is directed through the tissue and is detected by the photo-transistors on the opposite side. By comparing the ratios of the light absorbed by the tissue at each wavelength, the fraction of HGB in the form of HGBO can be determined.
However, surface oximetry is quite often unreliable or ineffective in hypothermic or peripherally vaso-constricted patients. Vaso-constriction is a common symptom of shock. Consequently, oximetry has been provided as a feature of some models of pulmonary artery catheters. Systemic arterial oximetry has been reported or is available as well. Systemic or pulmonary arterial oximetry readings are often more reliable than those obtained by surface oximetry since measurements are directly from the blood itself. Unfortunately, arterial oximetry by its very nature, requires a much greater degree of medical invasiveness and a corresponding degree of risk to the patient.
Since the esophagus is a core body organ and presumably well-perfused except under the most extreme circumstances, it has been determined that the esophagus could be an ideal locus for pulse oximetry measurements. This is especially so for anesthesiologists and critical care physicians who intubate the esophagus more or less routinely to monitor heart and breath sounds as well as core body temperature. Additionally, they frequently manage patients with hypothermia or in shock. Thus, it is desirable to provide an esophageal probe having a pulse oximeter therein for monitoring oxygen delivery to the esophageal mucosal tissue.
It has further been determined that for an oximetric sensor to make accurate measurements in the esophagus, it must be located somewhere in the middle third, the upper third, or even the upper three-fourths of the esophagus as measured from the gastro-esophageal junction to the hypo-pharynx. This is because the circulation to and from the esophagus at the area comprising the lower third or lower quarter of the esophagus is, in part, portal venous flow and hepatic venous flow or systemic venous flow and portal venous flow. In either case, because the flow is a mixture of portal and venous blood, it has a higher oxygen tension and therefore oximetric readings from the lower third or lower quarter of the esophagus would not provide a representative picture of cardiac function or perfusion to other body tissues.
Thus, it is desirable to provide an esophageal probe to monitor heart sounds, breath sounds, and body temperature which could include a pulse oximeter for measuring the fraction of HGB in the form of HGBO.
It is further desirable to provide an esophageal probe that can operate to locate the pulse oximeter in the upper two thirds or three fourths of the esophagus and still provide the various other functions for which it is intended.